She told Sky News: "I'm incredibly shocked. If this is about knee-gate, him touching my knee 15 years ago and me not having any issue with it at all, this is the most absurd resignation of a Cabinet minister ever.

"However I don't think that is the reason.

"I doubt very much if one of the standards they require in the armed forces means that no man in the military has ever touched the knee of a younger woman."

Jeremy Corbyn told Sky's Jon Craig the resignation news was "amazing". When asked if he would do an interview, the Labour leader commented: "Not yet."

Former deputy leader Harriet Harman also wrote on Twitter: "Well done, Theresa May."

While Lib Dem MP Alistair Carmichael said the Cabinet was "falling apart like a cheap piece of flat-pack furniture".

But Tory backbencher Nadine Dorries tweeted: "Fallon should stay as SoS. Another role. He expects the best from our armed forces. Seems to feel he's let them down. Job swap."

Sir Michael became Defence Secretary in July 2014, after ministerial roles in the Treasury, and Education and Energy departments spanning four prime ministers

He became MP for Darlington in 1983, but failed to win re-election nine years later. He then moved to Sevenoaks, the seat he has represented sine 1997. His majority was 22,000 at the June 2017 general election.